1. Field of the Invention
The present invention relates to a method for manufacturing of a graded index type plastic optical fiber and a manufacturing apparatus therefor.
2. Description of Background Information
In order to manufacture this type of plastic optical fiber, several methods have been disclosed, for example, in Japanese Published Patent Applications Nos.HEI 5-507610 and HEI 7-27928.
According to the method described in Application No. HEI 5-507610, a cladding is formed of a cylindrical tube having a hollow central part which is filled with a liquid mixture including a monomer and a compound having a high refractive index. The liquid mixture is then heated or irradiated with energy-carrying rays, thereby generating and propagating a polymerization reaction, preferentially on a predetermined site. The compound having a high refractive index thus forms a density gradient and is used as base material. The base material is melted by heating and is elongated so as to vary its refractive index continuously in the radial direction. This base material constitutes the core part of the plastic optical fiber.
In the method disclosed in Application No. HEI 7-27928, a polymerized fiber material is melted by heating and spun to form a fiber. The fiber thus obtained is soaked into a solution containing a non-polymerizable compound having a refractive index lower than that of the initial polymer, whereby this non-polymerizable compound is impregnated and diffused inside the fibers. Thus, a plastic optical fiber having a graded refractive index is obtained.
However, in the method disclosed in Application No. HEI 5-507610, the fiber is formed by heating, fusing and drawing the base material. The length of the fiber which can be produced from one base material is thus limited, i.e., the fiber length depends on the size of the base material.
In order to manufacture a longer fiber, the base material must have a greater diameter or must be of a longer size. When the base material is thickened, the subsequent drawing process becomes difficult because of insufficient heating and fusing. When the base material is lengthened, the material should have a uniform graded index and constant transmission loss in the longitudinal direction. However, the manufacture of such a material requires stringent conditions to be satisfied for controlling the reactions and polymerization. When manufacturing a plastic optical fiber having a length over 1,000 m and a diameter of 1 mm, a one-meter base material would have to have a diameter over 30 m.
Further, this method is a batch system in which a base material is first prepared, drawn and transformed into an optical fiber. The running ratio of the process therefore is not good and, as the production increases, the manufacturing facilities must be scaled up greatly.
In the method disclosed in Application No. HEI 7-27928, the fiber is manufactured continuously, so that it can be made with a desired length. However, after the fiber is manufactured, it has to enable a non-polymerizable compound to diffuse from its surface inwards. A large molecular-size, low refractive-index compound is, however, difficult to diffuse into such a fiber, so that the refractive index is poorly graded.
In order to diffuse a large molecular-size, low refractive-index compound into the fiber, the compound and the fiber must be heated to a high temperature. However, strong heating tends to melt and cut the fiber, and the fiber manufacturing becomes inconsistent.
In view of the above, an object of the present invention is to provide a novel method of manufacturing a graded-index type plastic optical fiber and a manufacturing apparatus therefor. This method attains a good running ratio of the process and minimizes the scaling-up of installation required for production increase. Further, the plastic optical fiber is easily provided with a desired graded index. Moreover, the graded-index fiber can be manufactured, continuously and constantly, in a desired length with a constant quality.